One common garden myth is trees have long tap roots extending straight down into the ground.  This is a common misconception of root growth, an incorrect perception shaped by “common sense” assumptions, not scientific evidence.  Most of us have seen seedlings germinate and immediately grown a “tap root” into the soil.  The root growth occurs prior to the seedling growing new leaves.  Once the tap root is established, new leaves begin to appear.

People generally think the tap root continues to grow downward, in essence resembling an enlarged carrot or a Christmas tree form.  The misconception of the tree crown and root system mirroring each other is commonly depicted in drawings of a lollypop shaped tree with identically shaped root system. 

Another misconception is tree roots stay beneath the tree canopy, only growing out to the drip line of the tree.  The misconception is excurrent shaped trees (holiday form Xmas tree, pyramidal shaped) grow similar shaped narrow, pyramidal tap root system, while decurrent, broad shaped tree crowns, (shade tree form), must have a more branched root structure, resembling a giant broccoli cluster.

Humans like symmetry, the perception that the root system mirrors the crown form is commonly depicted in tree renderings. Where the crown and root system mirror each other.  This is also seen in fertilizers with directions to apply near or at the “drip line”, the outer edge of the tree crown from which rain-water drips, as if this represents the majority of the tree’s absorbing roots.

Why does a tree grow a taproot?

Certified arborists know the taproot structure is a juvenile feature with specific functions important for a young tree, including:

• Anchoring a young tree
• Creating a vertical structure for lateral root development
• Storage mechanism for sugars
• Serves as part of the transport system for conducting nutrients and water to the trunk and canopy.

Root growth is dependent upon three factors for survival: water, oxygen and nutrients.  If any one of the three factors is missing, root growth and development stops.  Downward taproot growth slows as oxygen becomes increasingly limited with soil depth.  Compacted soils also are oxygen deficient due to crushed or damaged soil structure, limiting oxygen within soil micro and macro pores.  Flooded and or poorly drained soils are also oxygen deficient, with the same limiting effect on taproot and regular root growth and development.

As a taproot encounters limiting soil conditions, it begins to branch, adding lateral root development to explore soil resources within the top 12-18” of the soil.

Certified arborists understand how construction activity compacts the soil, thereby compressing the soil structure, damaging soil porosity resulting in limited oxygen, water and nutrient availability. As a result, urban tree maturation is dependent upon lateral root development, most of the structural rooting occurs within the top 12-18” of the soil.  It is estimated a trees functional root system extends two to three times the crown diameter at its widest point.  The juvenile taproot is subsumed by the rapidly developing lateral root system.

Understanding the root system location and lateral structure within the top 12-18” of the soil profile is vital to managing irrigation and maintenance practices.

Structural roots emanate from the trunk, they hold the tree upright but do not absorb any water or nutrients.  The fine root hairs responsible for water, oxygen and nutrient uptake may be located a considerable distance from the tree trunk, often at and beyond the edge of the tree crown, known as the drip line and beyond. Certified arborists and horticulturists understand proper irrigation methods, such as avoiding applying water on the root collar, designing water distribution where feeder roots are located beneath the  crown, toward the drip line and beyond.

In summary, Southern California soils do not support deep taproot development.  Construction and development compact soils, resulting in reduced soil porosity, water, nutrient and oxygen retention, three vital factors required for root development.  Over 80% of a tree root system is within the top 12-18” of the soil.  Taproot formation is a juvenile feature that is replaced by a horizontally growing lateral root system that extends two to three times the crown diameter.  Water and nutrients are taken up through the fine root hairs located beneath the crown, near and beyond the tree drip line.

Consider using  a certified arborist, horticulturist or landscape contractor, industry professionals who utilize industry standards, plant science, personal knowledge, experience and education to promote plant health.